1. Field of the Invention
This invention relates to electronic balun and mixing circuits and particularly to circuits designed for implementation with planar monolithic integrated circuit technology. This invention specifically relates to balanced mixers useful in radio frequency receiver systems, particularly rf, microwave and millimeter-wave frequency receivers.
2. Description of the Prior Art
Balanced mixers are required in many devices. When used in microwave receiver system applications, balanced mixers require wide dynamic range, low intermodulation products and effective local oscillator (Lo) AM noise suppression. A balanced mixer consists of two key subcomponents: a balun for achieving 180.degree. phase split and a frequency mixer for combining the input signals to create an IF or intermediate frequency signal. In an ideal balanced mixer, the output signals include only the sum and difference frequencies, that is, a pair of signals equal to the sum and to the difference in input frequencies.
Conventional balanced mixers commonly employ passive baluns configured from many different devices for providing the 180.degree. phase splitting, such as directional couplers, magic T's, rat race couplers, overlapped broadside couplers, Fin lines or even simply from a combination of an in-phase power splitter with unequal transmission lines. Such conventional baluns are limited by relatively large physical size, often approximately one half-wavelength long to obtain the 180.degree. phase shift, and relatively narrow bandwidths, usually less than a full octave in range. These limitations make such conventions designs undesirable for use in monolithic structures.